16. Assessment of Hydrologic Flux from the Haldi Catchment into Hooghly Estuary, India

Monitoring the impact of hydrologic flux from river basin toward the coastal environment is useful for assessment of terrestrial impacts on coastal ecosystem. In this study, hydrologic flux from the Haldi catchment into Hooghly estuarine system was simulated for the period 1989–1994 using semi-distributed watershed model, the Soil and Water Assessment Tool (SWAT) integrated with ArcGIS, i.e., ArcSWAT (2009.93.5). Sensitivity and auto-calibration analysis was also carried out to investigate optimal model parameters, while global sensitivity was carried out using SWAT-CUP Sequential Uncertainty Fitting Algorithm (SUFI2) module. Prior to calibration, observed stream flow was partitioned into two major components namely surface runoff and base flow using an automatic recursive digital filter technique. Base flow ranges between 13 and 29% of the observed stream flow during the period 1990–94. A detailed uncertainty analysis was carried out to assess the performance of SWAT model over the study area. Higher surface runoff is inferred from the enhanced calibrated parameter CN2, whereas reduction in the soil evaporation compensation factor (ESCO) was observed. This signifies the increased extraction of water from the lower soil moisture profile that results in meeting the requirements of evaporative demand. Thus, the output of the model is substantially affected by soil water capacity (SOL_AWC), where lower SOL_AWC implies reduction of soil retention capacity associated with increased surface runoff. Model performance indicator parameters such as R² and NSE show that the model performed satisfactory and good respectively at daily and monthly time scales.